Air conditioning units, which long have been prior art for motor vehicles, comprise refrigerant circuits with various individual components, like the condenser traditionally situated in the vehicle front end, the compressor connected to the vehicle engine and driven by it, the vaporizer situated in the passenger compartment, as well as hoses and connections. The air conditioner treats the air which then is directed into the passenger compartment. The compressor is usually driven from the engine of the motor vehicle by coupling the mechanical energy of the engine to the compressor shaft. Cooling fans and blowers are fed electrically from the 12-volt onboard network.
Heat is withdrawn at a high pressure level from gaseous refrigerant compressed in the compressor. In subcritical operation, the superheated refrigerant is cooled to the condensation temperature and then liquefied at a constant temperature. Thereafter, the completely liquefied refrigerant is further cooled in the condenser. The refrigerant is supercooled, with the supercooling relating to the constant condensation temperature. The range of the condenser in which the refrigerant is supercooled is also designated as the supercooling range. At the outlet of the condenser, the refrigerant usually exhibits a temperature that is about 5 K to 10 K below the condensation temperature.
In its installed position in the front end of the motor vehicle, the condenser is perpendicular to the air flow direction and for the most part has available a large network surface, which in small vehicles exhibits values in the area from 14 dm2 to 18 dm2; in vehicles in the compact class exhibits values in the range from 20 dm2 to 22 dm2; and in larger vehicles exhibits values above 24 dm2.
To be understood, a network surface is the surface at the entry or at the exit of the heat exchanger, essentially perpendicular to the flow direction of the air, also designated as the flow surface. The network surface comprises the ribbed area, or area configured with ribs of the heat exchanger, and it corresponds to the air-side flow cross section.
Traditional blowers of the condensers, also designated as cooling fans, are configured as axial fans and as suction fans at the outlet of a cooling module. Since axial fans are designed to deliver a large volume flow of air at a slight pressure difference, the heat exchangers situated in the cooling module, like the coolant cooler of the engine cooling circuit, the charge air cooler, or the condenser of the refrigerant circuit, are configured for reduction of the flow resistance with as small as depth as possible. The heat exchangers have air flowing through them one after the other on the air side. Depth is to be understood as the thickness of the heat exchanger in the flow direction of the air or the flow length on the air side.
The depth of the condenser known from prior art exhibits a value in the range from 12 mm to 16 mm. Due to the low length of flow on the air side and the large amount of air, the mass air flow is only slightly warmed when flowing through the entirety of the condenser. The mass air flow in the entry area of the condenser on the refrigerant side is considerably more heated due to the superheating of the refrigerant with temperatures above the condensation temperature than in the outlet area of the condenser on the refrigerant side, in which the refrigerant already is present in a condensed state and if necessary is supercooled.
Generic air conditioners with coolant-air heat exchangers, which relate to the heating performance from the refrigerant circuit of an efficient internal-combustion engine of the vehicle prime mover, at low ambient temperatures, for example lower than −10° C., no longer reach a temperature level required for comfortable heating of the vehicle passenger compartment. The same holds true for units in vehicles with a hybrid drive. For these vehicles, use of heating concepts is necessary.
Glycol-air heat pumps also use the coolant of the internal combustion engine, but as a heat source. With this, heat is withdrawn from the coolant. As a consequence of this, the internal combustion engine is run for a longer time at low temperature, which has a negative effect on exhaust emissions and fuel consumption. Due to the internal combustion engine operating intermittently in hybrid vehicles, during longer trips the coolant temperature does not become sufficiently high. As a consequence, the internal combustion engine is subjected to start-stop operation at low ambient temperature. The internal combustion engine is not shut off.
Additionally, there is a trend toward complete electrification of the drive train, as for example in vehicles driven by batteries or fuel cells. Here there is no waste heat from the internal combustion engine as a possible heat source for heating the air.
Furthermore, the amount of energy storable in the vehicle battery is less than the amount of energy storable in the form of liquid fuel in the fuel tank. Thus, the power needed to air-condition the passenger compartment of an electric vehicle has a considerable influence on the range of the vehicle.
In DE 10 2009 028 522 A1 a compact air conditioner is described with a vaporizer unit, a condenser unit, and a component unit as well as a refrigerant circuit. The vaporizer unit and the condenser unit both exhibit heat exchangers through which air flows, placed in a housing, as well as a blower. The refrigerant circuit, comprising a vaporizer, a condenser and a reheater, is configured for a combined cooler unit and heat pump operation, wherein in the reheater mode the heating power of the reheater is configured as a condenser-gas cooler and the cooling power of the vaporizer can be controlled independent of each other. The operational modes of the air conditioner are controlled by the refrigerant circuit. Thus, the air conditioner carries out the function of a heat pump, which is implemented by means of active switching within the one primary circuit and a secondary refrigerant circuit exhibiting a secondary branch formed from two flow paths. However, the configuration of the refrigerant circuit with switchover valves results in great complexity, which in turn leads to high costs and high technical expense.
From FR 2 743 027 A1 is derived a vehicle air conditioner with a traditional refrigerant circuit exhibiting only a vaporizer, a compressor, a condenser, and an expansion device. The heat exchangers are situated in separate flow channels, designed to be segregated from each other at least in flow terms. The flow channels exhibit cross connections or bypasses. The mass air flows brought in by suction using blowers are forwarded by closing and opening flaps as well as through passage via the bypasses, depending on the requirement and operating mode via the surfaces of the heat exchangers. The mass air flows are cooled and/or demoisturized or heated as well as then passed into the passenger compartment and/or the ambient environment.
Thus, air conditioners are known from prior art for vehicles for a combined cooling-unit and heat-pump operation with air as the heat source for heating, cooling, and demoisturizing the air to be fed to the passenger compartment and to be treated. The air conditioners are regulated either on the refrigerant-circuit side or the air side.
With the air conditioners controlled on the air side, however, no operation is possible in the reheating mode, also designated as Reheat. The air conditioners configured for additional reheating operation exhibit in turn a complicated refrigerant circuit with a plurality of components such as heat exchangers, switchover valves, and expansion valves.
In the Reheat or reheating mode the air to be fed to the passenger compartment is cooled and demoisturized in the process, then the demoisturized air is slightly heated up. In this operational mode, the required reheating power is at least less than the required cooling power for cooling and demoisturizing the air.
With the known air conditioners controlled on the air side, with a heat pump function, both in the cooling-unit operation and in the heat-pump operation, the vaporizer is operated as a vaporizer, and the condenser as a condenser. The heat flows are fully controlled via the air-side flow guidance. It is not necessary to switch the operation of a heat exchanger to the one as a condenser and to the other as a vaporizer.
However, condensers designed for heat-pump operation exhibit less transmission power than condensers designed for heating-unit operation. The condensers for heat-pump operation have less of a mass air flow through them and must cause a larger alteration of the air temperature.
According to the prior art, in heat pump systems, condensers are used with the structural space of a heat exchanger through which coolant of the engine cooling system flows. For this reason, these are configured as multi-row (for example, two-row) cross counterflow-heat exchangers with a designed depth of about 40 mm and a flow surface of about 4 dm2. Condensers with a designed depth of about 40 mm and a flow surface of about 4 dm2, when in operation as dual-row cross counter-flow heat exchangers, can heat a mass air flow in the range of 250 kg/h to 400 kg/h to a temperature which is about 5 K to 15 K below the condensation temperature of the refrigerant.
If the heat exchanger is run both in the cooling-unit operation and in heat-pump operation as a condenser, arrangement of the condenser in the cooling module of the motor vehicle does not make sense.
Due to the large structural shape of condensers for the cooling unit operation as known in prior art, i.e., with a large flow cross section on the air side, it is almost impossible to place the condenser in an area in the vehicle other than the cooling module.
On the other hand, with one designed in the structural form of a condenser configured for heat-pump operation, the required power for cooling unit operation is not transferable. In addition, the mass air flow cannot be heated to, or at all above, the condensation temperature of the refrigerant.